From the EP A-0 104 922 it is known to control a fine, low-power electron beam of a lithographic apparatus according to a computer stored map across the area of an integrated circuit to picture a pattern thereon.
Of said stored map, regions which will necessitate smaller accuracy of picturing are separated from regions necessitating higher picturing accuracy. The electron beam is moved with constant cross-sectional area over the surface areas of said integrated circuit which necessitate the same picturing accuracy. Thus, the cross-sectional area of the beam need not be changed at all subsequent beam positions.
An integrated circuit surface presents a small, flat and time-constant working surface for the electron beam for a lithographic process which electron beam has thus only to be deflected by small angles. Further, the effect of the electron beam impinging on said integrated circuit surface, is clearly predeterminable, the beam impinges normally once on every IC-area to be patterned.
In contrast thereto, the present invention is directed to evaporation of an area which presents a large, unsteady profile and an evaporation surface varying in time. In the inventive context, the electron beam must be deflected by relatively large angles according to the relatively large area to be evaporated and, to provide for an evaporation of material of said area, the electron beam has to be a relatively high-powered electron beam. Further, the instantaneous effect of the electron beam on the area to be evaporated is practically not predictable, due to the most complex and time varying structure of the surface being evaporated.
From the German laid-open print 3 428 802 a method for controlling the average evaporation rate as mentioned above is known, wherein there is assigned to the predetermined area to be evaporated a map with a position pattern for the electron beam. The electron beam is moved step by step from one position to the other according to the positions of the pattern of the map. Each position element of the position pattern which becomes controlling the electron beam position during its scanning work across the area to be evaporated, has assigned to it its proper control or correction value, influencing an operating parameter for the electron beam which influences the instantaneous evaporation rate of the beam on the predetermined area. Thus, the number of elements of the position pattern equals the number of control values which are assigned to the elements of the position pattern. The control values assigned to the respective position elements of the position pattern thus form together a control value pattern which is assigned to the predetermined area to be evaporated. With these correction values operating parameters of the electron beam are controlled, as e.g. the time during which the electron beam holds a predetermined position which is defined by an element of said position pattern or e.g. the focus of the beam at such a position or e.g. the power of the electron beam at such a position.
To set the proper correction values assigned to every element of the position pattern, the evaporation plant is calibrated and thereby the control values are adjusted, so that finally a desired evaporation rate distribution results across the entire area to be evaporated. Reaching of such a predetermined and desired evaporation rate distribution is necessary to evaporate a predetermined material according to predetermined criteria, so e.g. everywhere equally within a vacuum evaporation chamber of the plant.
This known method has the following disadvantages:
The calibration process of the plant is very time-consuming, because for each element of the position pattern the respective correction value must be evaluated.
Further, the storage amount is very large, because every correction value is to be stored, the number of which according to the number of position elements of the position pattern.
Further, such a beam control process becomes normally relatively slow, because at every change of position of the electron beam from one position pattern element to another, the new correction value according to the new position element must at least be read from the store and must possibly become active on the electron beam control. The new correction value must be read from the store in every case, even if it later turns out that the new correction value has the same value as the correction value which is effective at the instantaneous beam position.